Coal gasification burners offer a viable alternative to flue gas scrubbing for the utilization of high sulfur coals in a commercial utility steam generator. Coal gasifiers, in conjunction with combined cycle power generation, provide a significant decrease in the plant heat rate, resulting in cost savings in the production of electricity. One of the most attractive coal gasifier designs is an entrained upward gas flow unit firing pulverized coal to produce a low Btu and medium Btu product gas. Gasification at high pressure (greater than 14.7 psia) offers some additional advantage in reducing plant heat rate.
Operation of high temperature, high pressure chemical reaction processes has long presented a vexing problem in the chemical and heat transfer arts. Reaction vessels may be designed to withstand either high temperatures or high pressures, but not both simultaneously. One potential solution to this problem is the use of a double-wall containment vessel. A double-wall containment vessel is, in practice, two vessels, one wholly disposed within the other, and each designed to cooperatively contain a high temperature, high pressure reaction.
We will presently consider a double-wall containment vessel which includes an inner vessel, constructed with water-cooled membrane walls, for withstanding a high temperature reaction process. These membrane walls, constructed by longitudinally welding a plurality of substantially parallel tubes together, is, itself, unable to withstand a high pressure differential from one side to the other. Pressure differentials in excess of fifty inches of water (12.4 kPa) are generally considered to be the maximum accommodated by such membrane walls.
The outer vessel of a double-wall containment enclosure is designed to accommodate the high pressure of the process. The outer vessel is typically a solid vessel, possibly constructed of a high strength alloy and having a thickness of several inches. During start-up, load changes, and shut-down periods, a difference in temperature between the two suspended vessels cause their thermal expansions and contractions to differ significantly. For example, during start-up, the heat generated in the inner vessel will cause the temperature of the inner vessel walls to differ from the temperature of the outer vessel. The thermal expansion of the inner vessel will be manifested by a downward elongation expansion of several inches. Over a period of time, the temperature of the outer vessel will approach that of the inner vessel, accompanied by an elongation which will closely match the elongation of the inner vessel. These changes which are relative expansions longitudinally between the two vessels cause severe stress in the conduits for fuel and air as they penetrate the walls of the two vessels.
One high pressure, high temperature process is the pressurized conversion of solid coal particles into combustible fuel gas. A high pressure gasification process may require pressures in excess of five atmospheres, and temperatures in the hottest portions of the reactor up to 3000.degree. F. or higher. The introduction of abrasive coal into a double-wall reaction vessel has proved to be a problem due to the nature of the material being fed, and the interaction between the inner and outer vessel due to differential thermal expansion. Due to the nature of the chemical process, the fuel gas in the reaction vessel is hot and particle laden. In order to protect the outer vessel from this hot particle-laden gas, any penetrations through the wall of the inner vessel must be sealed.
The inner, heat resistant vessel, typically experiences a significant variation in dimension as it goes from an inactive state with temperatures corresponding to the surrounding environment, to the operating state, with coolant temperatures in the range of 600.degree. to 800.degree. F., and interior wall metal temperatures even higher. The outer vessel, possibly constructed of different material and well protected from the high temperature reaction by the inner wall, experiences a much slower rate of thermal expansion. The differential expansion may be as great as one foot (0.3 m) or more in a direction colinear with the longitudinal dimension of the inner and outer vessels. Such expansion must be accommodated by any feed line passing through the outer vessel wall and connecting to or passing into the inner vessel wall. One method of the prior art calls for the use of an expansion loop, disposed in the annular region formed between the inner and outer vessel walls, the expansion loop allowing the end points of a feed conduit to move each relative to the other without causing unacceptable stress in the looped conduit.
The feed of abrasive coal into the interior of the inner vessel has proved troublesome in that the expansion loops of the prior art are not able to withstand the high abrasion of the coal particles which occurs in the elbow sections of the looped conduit. Feeding coal is best accomplished through the use of a straight, rigid pipe which does not contain any significant bends. Such a straight, rigid pipe secured to the outer vessel wall and penetrating the inner vessel wall and sealed thereto has proved to be unsatisfactory due to the flexure of the feed conduit possible in the small annular region between the inner and outer walls.